Many chemical analysis applications use one or more sample tubes to collect, concentrate, and transfer a representative sample of a material to an analysis device. For example, gas chromatography involves vaporizing a sample and injecting the vaporized sample onto the head of a chromatographic column. The column is one of several fluidic components in the instrument. The sample is transported through this column by the flow of an inert gaseous mobile phase, also referred to as a carrier gas. The chromatographic column contains a liquid stationary phase that is absorbed onto the surface of an inert solid. The organic compounds in the sample are separated due to differences in their partitioning behavior between the mobile gas phase and the stationary phase in the column. Since the partitioning behavior is dependant on temperature, the separation column is usually contained in a thermostat-controlled oven. Separating components with a wide range of boiling points is accomplished by starting at a low oven temperature and increasing the temperature over time to elute the high-boiling point components. A detector is used to determine what compounds elute from the sample and a recorder provides the output in the form of a chromatograph.
Each of the fluidic components that comprise the instrument's fluid flow paths needs to be inter-connected to create an overall fluid flow network. Furthermore, many of these fluid connections need to be repeatedly disconnected and reconnected for several reasons, including maintenance of the instrument and system reconfiguration. Therefore, a reusable, leak-tight fluid connection or coupling is desirable.
There are a variety of fluidic sealing devices that can provide this type of fluidic connection. Among these devices are metal fittings and ferrules, gaskets and o-rings. Specific geometry and applications where o-ring seals are used can further characterize an o-ring fluidic connection as static, dynamic, radial, face and combinations of these. One of the most common, because it is simple and economical, is a static face-seal o-ring. In addition to low material and assembly cost, static face-seal o-ring connections provide several advantages over other common fluid sealing devices, such as being compact, simple, reusable and durable. When properly applied, static face-seal o-ring connections typically leak no more than 1.8×10−5 std. cc/sec Helium at 300 psig.
Unfortunately, o-rings in face-seal applications suffer from several limitations. They will leak if either sealing surface is rough, scratched or contaminated with particles or fibers. Surface finishes of 32 microinches RMS with no scratches, gouges or other imperfections are typically required to ensure a fluid-tight seal. In addition, o-rings are subject to physical damage during their manufacture, storage, handling, shipping and assembly into their sealing detail which may ultimately result in seal failure. Finally, particulate and fiber contamination easily adhere to the o-ring surfaces, which can affect seal performance. Special cleanliness requirements exist to achieve typical desired seal performance. These limitations result in less than 100% yield, especially in a fluidic network containing multiple fluid connections, such as in the assembly of a gas chromatograph instrument. The cost of troubleshooting and repair motivates the search for an improved fluidic coupling.